Considering a usual electromobile, it will be noted that the power which is available for the running of the electromobile is only the power which is stored by a charged battery having a limited capacity. Accordingly, it is necessary to make an effective utilization of the available power on a vehicle in order to increase the distance which is permitted by a single charging operation. At this end, a regenerative braking is very effective means in an electromobile. Thus, during such braking operation, the kinetic energy of the vehicle is used to drive an electric motor which is connected to wheels of the vehicle, thus returning the power generated by the motor to the battery, and thus reducing a wasteful power dissipation.
However, the braking effort which is obtained by the regenerative braking is limited, and accordingly to supplement it when a braking effort in excess of that available by the regenerative braking is required, an oil pressure brake is used in combination with the regenerative braking.
In a general braking system for automobile, a distribution between a front wheel braking effort and a rear wheel braking effort is determined so that a response curve, commonly referred to as an ideal brake force distribution, can be approached as close as possible. However, for an electromobile, if a distribution of oil pressure brakes associated with driven wheels is chosen high, there results a reduced efficiency of power recovery by the regenerative braking.
To accommodate for this, there is proposed a special mode, referred to as regeneration preferential mode, which establishes a distribution between the front and the rear wheel braking effort, which is different from the ideal brake force distribution, in order to increase the efficiency of power recovery by the regenerative braking.
For example, Japanese Laid-Open Patent Application No. 161,210/1993 discloses a regeneration preferential mode in which only the regenerative braking is carried out by disabling all the oil pressure brakes in a region where the required braking effort remains relatively low. Also, Japanese Laid-Open Patent Application No. 161,212/1993 discloses an automatic control in which a regeneration preferential mode normally prevails while a conventional mode according to the ideal brake force distribution is entered when a wheel locking is detected as a result of reduced coefficients of friction presented by road surface. Additionally, Japanese Laid-Open Patent Application No. 161,213/1993 discloses an automatic switching between a regeneration preferential mode and a normal mode in accordance with the force with which a brake pedal is depressed or a rate of an increase in such force of depression.
When the oil pressure brakes are disabled during the regeneration preferential mode, on/off valves which are interposed in flow paths disposed between a master cylinder, which generates an oil pressure in accordance with a stroke of depression of a brake pedal, and wheel cylinders disposed adjacent to the individual wheels are closed. When the brake pedal is depressed to a degree while the on/off valves remain closed, the oil pressure from the master cylinder increases in accordance with the stroke of depression, but since the wheel cylinders are maintained at low oil pressures, a large pressure differential is developed therebetween. If the on/off valves are opened under such condition to switch from the regeneration preferential mode to the normal mode, the oil will instantly flow into the wheel cylinders through the on/off valves until the oil pressure in the wheel cylinders approach the oil pressure in the master cylinder. Accordingly, there occurs a temporary depression in the master cylinder pressure (M/C pressure) as shown in FIG. 6, and the stroke of the brake pedal will rapidly change and an oscillation is produced in the force which depresses the pedal, as indicated in FIGS. 8 and 10. This degrades a braking feeling experienced by a driver, and may increase the distance over which the vehicle must be braked.